Monopole mass spectrometer having one ceramic electrode coated with metal to within a short distance of each end



3,350,559 ELECTRODE Oct. 31, 1967 J. R. YOUNG ETAL MONOPOLE MASS SPECTROMETER HAVING ONE CERAMIC COATED WITH METAL TO WITHIN A SHORT DISTANCE OF EACH END Filgd Jan. 2 T

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M! w (.5 o H w 3 I 3 3 2 Mm 7 H 2 mm 2 n .f. L 7 rl n u M INVENTORS: JAMES R. YOUNG, JOHN B. HUDSON, ROBERT L.WATTERS, A BY M THEIR ATTORNEY.

United States Patent James R. Young, Rexford, and John B. Hudson and 7 Robert L. Watters, Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Filed Jan. 26, 1965, Ser. No. 428,055 3 Claims. (Cl. 250-419) ABSTRACT OF THE DISCLOSURE A monopole mass spectrometer in which one of the electrodes is composed of ceramic material which material is metalized to within a short distance of each end.

This invention relates to mass spectrometers generally, and more particularly, to a monopole mass spectrometer having an improved analyzing tube electrical field.

Mass spectrometers are well known devices which are particularly applicable for use as partial pressure analyzers or for diiferent gas determination. In general, a mass spectrometer operates to analyze gas ions in a precisely determined uniform electrical field where, depending on the mass of the ions, the ions describe different trajectories. These ions may then be collected at different positions, based upon their ion trajectories, and the current generated thereby is a measurement of certain gas characteristics which may include the kind of gas, density of the gas, et cetera.

While several types of mass spectrometers are commercially available, one preferred form is denoted as a monopole mass spectrometer, an example of which is disclosed and described in the Review of Scientific Instruments, vol. 34, 1963, p. 1, U. von Zahn. A monopole mass spectrometer employs an R-F electrical field between a parallel spaced apart two-electrode configuration to analyze ions of different e/m. One axially extending electrode has a right angle cross section and the other axially extending electrode has a circular cross section whose center is located on the bisector of the right angle. In a monopole partial pressure analyzer, ions of the gases being analyzed are injected into the mass analyzer region of the analyzer tube by a rather conventional Nier-type ion source. After passing through an aperture the entering ions are subjected to a combined D-C and R-F electric field. Solutions to the equations of motion of charged particles in such a fiel-d (Mathieu functions) indicate that for a given set of operating parameters (D-C voltage, R-F voltage, frequency and field radius) particles of only a small range of charge to mass ratio will have stable trajectories and be able to pass through the analyzing region. Particles of all other charge to mass ratios will have unstable trajectories and thus will be filtered out of the entrant ion beam before reaching the ion detector.

In utilizing and testing monopole mass spectrometers as described, it was noted that at very high imposed voltages, or at very low pressures, the resolution and accuracy was seriously deficient for no apparent functional reason. This noted deficiency is a serious limitation to the operating range. Further investigation of this problem resulted in the discovery that changes were necessary in the analyzer tube portion, particularly in the electrical field, in order to overcome the stated deficiency.

Accordingly, it is an object of this invention to provide an improved monopole mass spectrometer.

It is another object of this invention to provide an improved electric field for a monopole mass spectrometer.

It is yet another object of this invention to provide a 3,350,559 Patented Oct. 31, 1967 physical and electrical interference free electric field for a monopole mass spectrometer.

It is yet another object of this invention to provide a pair of elongated parallel spaced apart electrodes to define an electric field in a monopole mass spectrometer where the electrodes are maintained in their spaced relationship at the ends thereof.

It is a still further object of this invention to provide a pair of elongated spaced apart parallel electrodes in a monopole mass spectrometer electrical field where the said electrodes are maintained in spaced apart condition at their ends by spacer blocks and gas ions to be analyzed pass through the magnetic field without extraneous interfering effects from these blocks or other adjacent structure.

Briefly described, this invention in one of its preferred forms includes a first axially extending cylindrical section electrode, and a second axially extending electrode of metal, in the form of a V-cross section. The described electrodes are positioned in interfitting yet parallel spaced apart relationship and supported in that manner by block supports at the end structures only. The end structure and assembly is so formed and arranged that ions to be acted upon by an electric field between the electrodes enter one end structure and thereafter do not see any obstruction within the electrical field or any electrical interference from a charge containing structure which would adversely affect their trajectory through the field from that which would ordinarily be caused by the field. These ions pass through the electric field and egress from the remaining end structure. V

This invention will be better understood when taken in connection with the following description and the figures in which:

FIG. 1 is a side elevational and partial sectional view of an analyzer tube of one preferred embodiment of this invention,

FIG. 2 is a partial and exploded view of one end portion of FIG. 1,

FIG. 3 is an end view of FIG. 1 taken along the line 3-3,

FIG. 4 is a further end view of the modification of FIG. 1.

Referring now to FIGS. 1 and 2, there is illustrated, in one preferred form, the analyzing tube portion 10 of a monopole mass spectrometer of the kind disclosed and described in the mentioned Review of Scientific Instruments article. A further article, R-F Mass Spectrometer Partial Pressure Analyzers by John B. Hudson, General Electric Company, also describes a monopole mass spectrometer. The analyzer tube 10 is employed in conjunction with an ion source and an ion detector which are not shown and do not constitute essential parts for the purpose of describing this invention. In addition, the structure as illustrated in FIG. 1 is adapted to be positioned within a stainless steel tube or envelope structure or otherwise define an enclosed evacuated structure. Analyzer tube 10 includes a pair of concentric spaced apart endplate or support members 11 and 12. Members 11 and 12 are maintained in their spaced apart position by, and in turn support, a pair of axially extending electrodes 13 and 14. Electrode 13, which in one preferred form is of a stainless steel material, includes a pair of rectangular block members 15 and 16 which are maintained in engagement in or right angle relationship (FIG. 2) to provide a V configuration. The V configuration is retained within the endplates 11 and 12 by means of suitable cap screws 17 passing through each endplate member 11 and 12 to secure each side or each leg 15 and 16 of the V configuration.

An axially extending R-F electrode 14 is also supported by the endplates 11 and 12 in parallel spaced relation to 3 electrode 13. More particularly, R-F electrode 14 comprises in one form a substantially arcuate member having a general semicircular or U-shaped cross section. Electrode 14 is positioned in parallel spaced relationship to electrode 13 and with the base of the semicircle in receiving relationship within the legs of electrode 13. As

7 such the upstanding portion of the arcuate member lies in generally conforming relationship to the upstanding leg members 15 and 16 of electrode 13. The described structure and relationship provides, between electrodes 13 and 14, an electrical field which in the transverse or axial direction is precisely uniform in action upon traversing gas ions. To provide the required electrical field, electrode 13 may be attached to a suitable source of power through a conductor attached to one of the end members 11 and 12 in a conventional manner. A spring finger kind of contact may be directly brazed or soldered to electrode 14 interiorly of the arcuate section to connect electrode 14 to a suitable source of R-F power.

In the analyzer tube of FIG. 1, the positioning or relationship of electrodes 13 and 14 must be of an extremely precise nature in order to obtain the required accuracy of the spectrometer device. Therefore, the materials utilized and their relative sizes are chosen to provide preciseness of assembly, operative electrical and vacuum characteristics and sutficient structural integrity. One preferred form of electrode 14 has been found to be a half section of a suitable metal rod or hollow cylinder. Electrode 14 may also take the form of a ceramic material which has been rnetalized or coated with a suitable metal on its inner and outer surfaces. For example, a high purity alumina such as Lucalox alumina may be coated with a platinum or other suitable electrically conducting metal to provide the electrode 14. However, the metal coating 18 as illustrated in FIG. 1 must terminate prior to the ends of the electrode 14, where each axial end of electrode 14 contains a portion 19 free of any metal coating. A relatively gas free high purity alumina material is substantially free from distortion due to thermal stresses and, where sufficient clearance is allowed to adjacent member, shows but minimal and negligible changes of position during bakeout or at high temperature operating conditions.

Since electrodes 13 and 14 are to be connected to a suitable source of electrical power to generate an electrical field therebetween, they must be suitably electrically insulated from each other. At the same time, electrodes 13 and 14 must be so supported that during temperature extremes of bakeout and operating conditions, the imposed thermal stresses will not be such as to change the precise spaced apart positioning and thus affect the accuracy of the instrument. For example, the spaced apart difference between electrodes 13 and 14 must be maintained within and less than 0.001 of an inch under operative conditions. It thus becomes desirable to utilize one of the electrodes, for example electrode 13, to position and at least partially support electrode 14 so that minor variations of one electrode are caused to vary both electrodes in unison, and the spaced apart dimensions are maintained accurate.

For example, suitable spacer blocks 20 may be located between electrode 13 and electrode 14 to maintain the close fitting and tight engagement therebetween under all operative conditions. When electrode 14 is of metal the blocks 20 are of an insulating material such as ceramic. Together with the use of a metal coated ceramic member for the electrode 14, where the ends thereof are uncoated, metal spacer blocks 20 may be utilized between electrode 13 and the uncoated ends of electrode 14 without danger of electrical conductance between the electrodes. While both metals and non-metals may be employed, a preferred metal block member is in the form of stainless steel of about A inch edge thickness. These spaced blocks are employed contiguously adjacent the endplate members 11 and 12 and between electrodes 13 and 14 to provide precise separation thereof. As illustrated in FIG. 2, a pair of blocks 20 are utilized at each endplate and lie along and between the arms of electrodes 13 and 14 generally intermediate the exposed length thereof. These blocks may be suitably attached to the end members 11 and 12 in a retained but somewhat sliding fit relationship to provide relative movement without affecting the accuracy of the spaced relationship. For example, as more clearly shown in FIG. 2, a single spring clip retaining means 21 is so employed.

With electrode 14 resting upon four blocks 20 compression means are employed to attach electrode 14 or fix it in the described position. As shown in FIGS. 1, 2 and 4, a kind of spring clip 22 is attached by screw means 23 to end member 11. An angled portion 24 of clip 22 including a cross bar 25 transversely overlies the uncovered end of electrode 14, thus forcing electrode 14 against blocks 20. A similar clip arrangement is also utilized for the other end of electrode 13.

The use of spacer blocks 20 in their described position is an important feature of this invention. Heretofore, certain block or spacer members were utilized within or adjacent the defined electric field, i.e., they were spaced from or inwardly from end members 11 and 12, and where electrodes 13 and 14 were of metal, the spacer blocks were of an electrically insulating ceramic material. It has been discovered that these spacers seriously reduce the resolution and low pressure accuracy of the mass spectrometer because of an electrical charge buildup on the blocks at high voltages. The charge buildup takes place at 2500 to 3000 volts peak and the adverse results are noted at operating voltage of 1000 volts. Inaccuracy is also indicated at pressures of 10- torr and lower. This charge causes local arcing during operation or the charge potential otherwise distorts or interferes with the necessary uniform field. It is therefore imperative that the analyzing portion of the electric field be maintained free of this kind of obstruction or interference. No charge can build up on metal blocks 20 which are adjacent metal end members 11 and 12, members 11 and 12 being connected to one side of a source of potential. By the same token, no charge would build up on a ceramic block in the same position. Electrode 14 may be made of metal and have ceramic end portions engaging metal blocks.

During operation of the analyzer tube 10, gas ions from the ionizer are caused to enter the axially defined space between electrodes 13 and 14 through one end member such as for example member 11. As illustrated in FIG. 1, member 11 includes entrance plate means 26 which includes a very small triangular opening 27 positioned concentric with an in forward spaced relationship to the apex of the V electrode 13. In FIG. 3, opening 27 is shown as having two of its sides parallel to and coextensive with the sides of electrode 13 at its apex. Plate means 26 is easily removable to change the kind and size of ion aperture 27. Referring again to FIG. 1, the ions entering aperture 27 progress through the defined space between electrodes 13 and 14 and egress from tube 10 through a similar plate means 28 having a corresponding aperture 29 therein. Because the ions may be acted upon by the electric field to change their trajectories, aperture 29 is made much larger than aperture 27.

In order to ascertain that the ions entering the defined electric field are not acted upon or exposed to extraneous electrical charges or other interferences, a suitable channel or tunnel member 30 is employed. Referring to FIG. 1, tunnel member 30 includes a metal such as for example stainless steel produced in an arcuate form to rest within the apex of the electode 13 and to form therewith a closed tunnel member having an entrance and exit. As better illustrated in FIG. 3, the tunnel member 30 is shown as a strip member bridging the apex of the electrode 13 from one arm 15 to the other arm 16 and providing suitable shielding means for the ions passing therethrough until such time as the ions enter the analyzing portion of the magnetic field at a point where they will be free from effects to any end support structure mounting of the electrodes 13 and 14. Tunnel member 30 may be suitably attached to end member 11 by means of a right angle strip 31 which is attached by welding to tunnel member 29 and end member 11. It is preferable that the tunnel member 30 extend axially Within the space defined between electrodes 13 and 14 a distance which is beyond the support blocks 20 is illustrated in FIG. 1. A similar tunnel (FIG. 4) is also provided at the other end of analyzer tube adjacent the endplate 12 to provide for egress of ions without encountering electric field obstructions. In accordance with the structure as described, the gas ions must enter the analyzer tube and progress therethrough and leave the tube, and in the interim period be acted upon by an axial uniform electrical field which is free from any extraneous disturbance or effect caused by structure therein which develops an electrical charge when the field generated by high voltages. Tunnel member 30 may be employed in lieu of structural relationship of the ion entrance and exiting means being such that, for example, entering ions do not see or are not effected by any end structure since they may be cause to enter well within the electric field.

It can thus be seen that the objects of this invention are attained in a monopole mass spectrometer by supporting a pair of axially extending electrodes in parallel spaced relationship by means of endplates, and having electrode spacer means closely adjacent the endplates. Additionally, the ion inlet and the ion exit which are spaced apart along the axial dimensions of the electrodes are so arranged that entering ions during their transversal of the magnetic field see only a uniform magnetic field and no electrical charge disturbances caused by structure within or closely adjacent the analyzing electrical field. The requirements defined all relate to the feature that there must be electric field forming and defining means which not only provide a uniform and precise field, but also which include no extraneous charge developing means or structure so related to the electric field as to have a measurable effect on ions passing therethrough.

While this invention has been described with reference to particular and exemplary embodiments thereof, it is to be understood that numerous changes can be made by those skilled in the art without actually departing from the invention as disclosed, and it is intended that the appended claims include all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A very high voltage low pressure monopole mass spectrometer analyzing tube comprising in combination,

(a) a pair of endplate spaced apart support members,

(b) a V-shaped electrode extending axially between and supported by said endplate members,

(c) a metal coated ceramic electrode extending between said plate members in parallel and precise spaced relationship with said V electrode,

(d) said ceramic electrode having the ends thereof uncoated,

(e) said ceramic electrode being arcuately shaped in cross section and positioned in conforming and confining relationship with said V electrode,

(f) said electrodes adapted to be connected to a source of electrical power to define therebetween an electrical field which is uniform axially,

(g) metal support means to precisely support said electrodes in parallel spaced relationship,

(h) said metal support means being positioned adjacent said end support members and in contact with an uncoated portion of said ceramic electrode,

(i) ion entrance means in said endplate support members to introduce ions into said electric field,

(j) ion exit means in the other of said endplate members to exit ions from said field.

2. The invention as recited in claim 1 wherein shielded tunnel means are employed in said end members for the entering and exiting ion beams.

3. The invention as recited in claim 1 wherein said metal support means includes four metal blocks, a pair at each end of said electrodes and positioned between the upstanding arms of said electrodes.

References Cited UNITED STATES PATENTS ARCHIE R. BORCHELT, Primary Examiner.

RALPH G. NILSON, Examiner.

A. L. BIRCH, Assistant Examiner. 

1. A VERY HIGH VOLTAGE LOW PRESSURE MONOPOLE MASS SPECTROMETER ANALYZING TUBE COMPRISING IN COMBINATION (A) A PAIR OF ENDPLATE SPACED APART SUPPORT MEMBERS, (B) A V-SHAPED ELECTRODE EXTENDING AXIALLY BETWEEN AND SUPPORTED BY SAID ENDPLATE MEMBERS, (C) A METAL COATED CERAMIC ELECTRODE EXTENDING BETWEEN SAID PLATE MEMBERS IN PARALLEL AND PRECISE SPACED RELATIONSHIP WITH SAID V ELECTRODE, (D) SAID CERAMIC ELECTRODE HAVING THE ENDS THEREOF UNCOATED, (E) SAID CERAMIC ELECTRODE BEING ARCUATELY SHAPED IN CROSS SECTION AND POSITIONED IN CONFORMING AND CONFINING RELATIONSHIP WITH SAID V ELECTRODE, (F) SAID ELECTRODES ADAPTED TO BE CONNECTED TO A SOURCE OF ELECTRICAL POWER TO DEFINE THEREBETWEEN AN ELECTRICAL FIELD WHICH IS UNIFORM AXIALLY, (G) METAL SUPPORT MEANS TO PRECISELY SUPPORT SAID ELECTRODES IN PARALLEL SPACED RELATIONSHIP, (H) SAID METAL SUPPORT MEANS BEING POSITIONED ADJACENT SAID END SUPPORT MEMBERS AND IN CONTACT WITH AN UNCOATED PORTION OF SAID CERAMIC ELECTRODE, (I) ION ENTRANCE MEANS IN SAID ENDPLATE SUPPORT MEMBERS TO INTRODUCE IONS INTO SAID ELECTRIC FIELD, (J) ION EXIT MEANS IN THE OTHER OF SAID ENDPLATE MEMBERS TO EXIT IONS FROM SAID FIELD. 